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Cellulose
Polysaccharide
Fructose
Monosaccharide
Sucrose (table sugar)
Disaccharide
Glycogen
Polysaccharide
Glucose
Monosaccharide
Galactose
Monosaccharide
Ribose
Monosaccharide
Lactose (milk sugar)
Disaccharide
nucleophilic substitution, oxidation-reduction, and rearrangements
These mechanisms in Carbohydrate Chemistry are types of chemical reactions that carbohydrates undergo to form different structures and functions.
Mutarotation of Monosaccharides
Concept: Monosaccharides exist in equilibrium between their open-chain and cyclic forms. In aqueous solution, they undergo mutarotation, where the α- and β-anomers interconvert via the open-chain aldehyde (or ketone) form.
Mechanism:
The hemiacetal/hemiketal bond opens to form the open-chain aldehyde (or ketone) intermediate.
The molecule can then cyclize again, leading to a different anomeric form.
This process is catalyzed by acid or base.
Glycosidic Bond Formation (Acetal Formation)
Concept: A glycosidic bond forms when the hydroxyl group of one carbohydrate reacts with the anomeric carbon of another carbohydrate under acid catalysis, leading to the formation of disaccharides or polysaccharides.
Mechanism:
Protonation of the anomeric hydroxyl group increases its leaving group ability.
The departure of water generates a carbocation (or an oxocarbenium ion).
A second carbohydrate nucleophile attacks the anomeric carbon.
Deprotonation results in the formation of a stable glycosidic bond.
Oxidation of Monosaccharides
Monosaccharides can be oxidized at different positions:
Aldonic acids (oxidation at C1) → Example: Glucose to gluconic acid.
Uronic acids (oxidation at C6) → Example: Glucose to glucuronic acid.
Aldaric acids (oxidation at both C1 and C6) → Example: Glucose to glucaric acid.
Mechanism (Glucose to Gluconic Acid):
The aldehyde form of glucose is oxidized by an oxidizing agent (e.g., Tollens' reagent, Benedict’s reagent).
The aldehyde group is converted into a carboxylic acid.
Reduction of Monosaccharides (Formation of Sugar Alcohols)
Aldoses and ketoses can be reduced to form sugar alcohols (alditols) by using reducing agents like sodium borohydride (NaBH₄).
Mechanism:
The carbonyl group (aldehyde or ketone) is attacked by hydride (H⁻) from NaBH₄.
This converts the carbonyl to a hydroxyl (-OH) group, forming a polyalcohol.
Example:
Glucose → Sorbitol (by reduction of the aldehyde).
Epimerization (Base-Catalyzed Isomerization)
Concept: A monosaccharide can undergo base-catalyzed enediol rearrangement, leading to epimers (differing at one chiral center).
Mechanism (Glucose to Mannose):
Base deprotonates the α-hydrogen of the aldehyde form of glucose.
The enolate intermediate forms.
Reprotonation at a different position leads to the formation of mannose.
Benedict’s and Fehling’s Test (Detection of Reducing Sugars)
Reducing sugars (like glucose) react with Cu²⁺ in an alkaline solution to form a red precipitate (Cu₂O).
Mechanism:
The open-chain form of glucose reduces Cu²⁺ to Cu⁺.
Cu⁺ further precipitates as Cu₂O (red color).
Hydrolysis of Polysaccharides
Concept: Polysaccharides like starch or cellulose can be broken down into monosaccharides by hydrolysis.
Mechanism (Acid-Catalyzed Hydrolysis of Starch):
Acid protonates the glycosidic bond, making it more susceptible to nucleophilic attack.
Water attacks the anomeric carbon.
Cleavage of the glycosidic bond occurs, producing smaller sugar units.